It uses a single battery and according to "psst" from Aron's forum, it's the best sounding of them all. It's got the LFO of the Deluxe (including it's controls) and the signal path of the ORIGINAL Electric Mistress.

The Deluxe Electric Mistress flanger uses two batteries for 18V, but it regulates it down to 12v, so the difference in voltage between the 9v version and the "18v version" is really only 3 volts.

While it's possible that those 3 volts give a little more headroom, it's hard to believe it makes that much of a difference in the overall operation of the circuit, and having a single battery supply is a lot easier, not to mention the fact that you can power it with your other 9v pedals power supply.